Why are Moderna and Pfizer COVID-19 vaccine side effects worse after the second shot?

Why are Moderna and Pfizer COVID-19 vaccine side effects worse after the second shot?

Side effects from both the Moderna and Pfizer COVID-19 vaccines are more common after the second dose. This is because your immune system recognizes the virus spike protein from the first dose of the vaccine and mounts a stronger response. Oct 28, 2021